1. Field of the Invention
The present invention relates to a two stage turbocharger system for an internal combustion engine, and provided with two turbochargers arranged in series in the direction of the gas flow.
2. Description of the Related Art
Known in the prior art is a two stage turbocharger system for an internal combustion engine wherein two turbochargers are arranged in series in the direction of the gas flow, to obtain a desired performance of the engine over a wide operating range from a low speed to a high speed. In this case, one of the turbochargers is a small volume unit used for operation in the low speed range, and the other turbocharger is a large volume unit used for operation in the high speed range. Note, the small volume turbocharger is arranged near to the engine so that the exhaust gas from the engine is first supplied to the turbine of the small volume turbocharger and then to the turbine of the large volume turbocharger. Conversely, the air fed to the engine is first compressed by the compressor of the large volume turbocharger and then by the compressor of the small volume compressor.
In such a combination of small and large turbochargers, both units are constantly operated, which results in a lower efficiency during an engine high speed condition because energy is unnecessarily consumed. Therefore, to increase the efficiency, a system is provided wherein a bypass control device is provided so that both the small volume and the large volume turbochargers are operated during a low speed condition but only the large volume turbocharger is operated during a high speed condition, as disclosed in Japanese Unexamined Utility Model Publication No. 50-129815 and No. 59-82526.
The bypass control device is provided with a passageway bypassing the turbine of the small volume turbocharger, and a bypass control valve arranged in the bypass passageway and responsive to the intake pressure at the output of the small volume turbocharger. Accordingly, when the engine speed becomes high, the bypass control valve is opened and exhaust gas from the engine is made to bypass the turbine of the small volume turbocharger, resulting in an increased efficiency. The bypass control valve is constructed as a diaphragm valve having a diaphragm opened to the engine intake passageway downstream of the compressor of the small volume turbocharger, and a spring urging the diaphragm to close the valve. Accordingly, when a pressure of the intake passageway downstream of the compressor is higher than a preset value corresponding to the spring coefficient of the spring, the diaphragm is moved against the force of the spring. As the pressure is further increased, the force acting on the diaphragm to displace same becomes larger. Therefore, this type of bypass control valve has a linear characteristic between the degree of opening of the valve and the pressure applied to the diaphragm, and this linear characteristic is determined by the spring coefficient of the spring.
Nevertheless, this single stage spring bypass control valve suffers from a drawback in that there is a drop in the rate of increase of the turbo-pressure at an intercept point. This intercept point is defined as a point corresponding to the engine speed at which, the increase in the turbo-pressure begins to level off to the target turbo-pressure. The most desirable characteristic is that in which the pressure at the intercept point is equal to the target turbo-pressure, but it is difficult to obtain this characteristic with the usual single stage spring actuator, since this spring has only one spring coefficient value. If the spring coefficient value is made larger, the intake pressure can be higher than the desired intake pressure when the engine speed is high, but if the spring coefficient value is made lower, the valve is prematurely opened causing the pressure at the intercept point to fall.
An increase of the intake pressure at the intercept point is possible if the full stroke position of the intake by-pass control valve is not reached at the point at which the large volume turbocharger is fully activated. This, however, causes an excessively high rotation speed of the small volume turbocharger, which shortens the service life of the small volume turbocharger and lowers the fuel consumption efficiency, since the small volume turbocharger is operated unnecessarily.